Bruno may shed some light on this phenomenon.
The 727 with its highly swept- back wing has negative stability with both its yaw dampers turned off...the third oscillation, if allowed to continue, will go inverted.
One wing moving forward, relative to the other, will have more lift, relative to the other, so it "rises" relative to the other. But, its drag increases and so it retreats relative to the other, losing lift and "descends", relative to the other. Side to side and up and down, it gets interesting very quickly.
When fuel was cheap, pilots in training practiced controlling Dutch Roll manually in the airplane up around 35000' at high Mach numbers. In later years training was done in simulators, which proved more difficult to handle for some pilots, who broke out in hives and muttered things about Boeing engineers, so I've been told......
dg